Problem: 3 markers cost $5.79. Which equation would help determine the cost of 15 markers?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. We can write the fact that 3 markers cost $5.79 as a proportion: $\dfrac{3}{\$5.79}$ Let $x$ represent the unknown cost of 15 markers. Since 15 markers cost $x$ , we have the following proportion: $\dfrac{15}{x}$ The cost changes along with the number of markers purchased, and so the two proportions are equivalent. $\dfrac{3}{\$5.79} = \dfrac{15}{x}$